You might still enter a username, otherwise a random one will be used.

You can call other phone using it's full SIP URI:user@IP:port

The main reason of a registrar SIP server is to avoid this headache as in this case you have to find out the port and the IP can also change (on DHCP) and you have to type a long URI instead of just the username of the peer.

If I set the local signalling port to 50100 ie within the range of 49152 to 65534 and use the name in Mizudroid then john@192.168.50.95:50100 works as an address for a voice call. I'll have to play with messaging.

The idea of a local server is interesting and makes the solution scalable without changing the user interface.

The reason the LAN without a server is important is two phones and a GL-AR150 will work. The GL-AR150 can be powered from a external battery pack of the type used to recharge phones. Add in a small solar panel and life becomes interesting.